Process for the electrochemical preparation of gamma-hydroxycarboxylic esters and gamma-lactones

ABSTRACT

γ-Hydroxycarboxylic esters and γ-lactones which are suitable as flavors can be prepared by electrochemical reductive cross-coupling of α,β-unsaturated esters with carbonyl compounds in an undivided electrolysis cell having a cathode composed of lead, lead alloys, cadmium, cadmium alloys, mercury, steel, glassy carbon or boron-doped diamonds and a basic aqueous electrolyte comprising an electrolyte salt which suppresses the cathodic formation of hydrogen.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No.13/594,028, filed on Aug. 24, 2012, which incorporates by reference theprovisional U.S. application 61/526722 filed on Aug. 24, 2011, andclaims foreign priority to EPO 11178688.5 filed on Aug. 24, 2011, theentire content of which is incorporated herein by reference.

DESCRIPTION

The invention relates to a process for the electrochemical preparationof y-hydroxycarboxylic esters and γ-lactones by reductive cross-couplingof α,β-unsaturated esters with carbonyl compounds in an undividedelectrolysis cell, in which a cathode composed of lead, lead alloys,cadmium, cadmium alloys, mercury, steel, glassy carbon or boron-dopeddiamonds and a basic aqueous electrolyte comprising an electrolyte saltselected from among bisquaternary and multiquaternary ammonium andphosphonium salts are used.

The invention further relates to the y-butyrolactone derivatives of theformula I

which can be prepared by the process of the invention, and also theiruse as flavors.

The invention also relates to the γ-hydroxycarboxylic acids orγ-hydroxycarboxylic esters of the formula VII

which can likewise be prepared by the process of the invention.

The industrially most important γ-lactone is γ-butyrolactone. It isprepared industrially either by dehydrocyclization of 1,4-butanediol inthe gas phase or by hydrogenation of maleic anhydride. A furtherclassical method for preparing γ-lactones is the alkaline hydrolysis ofγ-halocarboxylic acids.

The above-described methods always go out from an existing disubstitutedC4 framework, so that substitution patterns on the ring cannot berealized convergently. However, methods in which the future lactone ringis built up only by means of C,C-bond coupling are also known. Theseinclude, for example, the oxidative coupling of acetic acid with olefins(C2+C2) or the tert-butyl hydroperoxide-aided cyclization of acrylicacid with alcohols (C3+C1). In these cases, the substitution of the ringcan be controlled by clever use of the appropriate starting materials inthe cyclization.

This type of lactone synthesis also includes the reductive coupling(dihydrodimerization) of acrylic esters and carbonyl compounds accordingto the following reaction scheme:

The reductive coupling of acrylic acid derivatives with carbonylcompounds can be effected by means of reducing agents such as magnesiumor samarium(II) iodide. Electrochemical methods which avoid thestoichiometric use of a chemical reducing agent have also beendescribed. Fundamental metal studies in this field were carried out in adivided electrochemical cell at a mercury pool cathode in a sulfuricacid electrolyte at cathodic current densities of up to 2.8 A/dm². Inthe studies, the reductive coupling of acrylonitrile with acetone wasobserved, which thus do not yet lead to the lactones.

Proceeding herefrom, Shono et al. (Tetrahedron Lett. 1980, 21,5029-5032) have described the reductive coupling of α,β-unsaturatedesters with aldehydes or ketones in a divided electrochemical cell. Theelectrolyte used was based on N,N-dimethylformamide (DMF) withN,N,N,N-tetraethylammonium toluenesulfonate (Et₄NOTs) as electrolytesalt. Furthermore, stoichiometric amounts of a chlorosilane(trimethylsilyl chloride, TMSCI) were added to activate the carbonylcomponent. The electrolysis was carried out at a current density of 0.4A/dm², which is far removed from industrially relevant current densitiesof >1 A/dm². Nobuya et al. (JP 57108274 A) have undertaken a furtherstep toward industrial implementation by using a water-basedelectrolyte. The preparation of the lactones was carried out in adivided electrolysis cell at current densities of 10 A/dm². Here, anacidic anolyte (e.g. 10% strength H2SO4) and a KH2PO4-buffered catholytewere used. In divided cells, the two electrode spaces are separated by amembrane. Undivided cells are cheaper and industrially easier torealize. Particularly in the case of organic processes, rapid aging ofthe membrane and therefore unsatisfactory operating lives can beexpected.

U.S. Pat. No. 4,414,079 describes the reaction of α,β-unsaturated esterswith aldehydes in an undivided cell using, for example,tetra-n-butylammonium sulfate as electrolyte salt. In a furtherapproach, Bürger (Katrin Bürger, Thesis 2003, Universitat Munster) hascarried out the reaction of α,β-unsaturated esters with aldehydes orketones in an undivided cell. Electrolytes used were binary mixtures ofalcohols (e.g. methanol or ethanol) with water or dioxane and also highconcentrations of electrolyte salts (e.g. tetrabutylammoniumtetrafluoroborate, Bu₄NSF₄). Interestingly, graphite electrodes wereused in the system described and, owing to their comparatively highhydrogen overvoltage, these could serve as alternatives for lead,mercury and cadmium electrodes. However, the yields of lactone which canbe obtained by this process are unsatisfactory since the correspondinghomo-coupling products and the reduced carbonyl component (i.e. thecorresponding alcohol) are formed to a large extent as by-products, eventhough the homo-coupling of the α,β-unsaturated esters is countered by ahigh excess of carbonyl compound. In addition, this process is based onthe use of electrolytes based on binary organic solvents (alcohol withwater or alcohol with dioxane), which makes a complicated separation ofthe product from the solvent necessary after the electrolysis. The useof alcohol-comprising solvents is also disadvantageous because thealcohol is oxidized (to aldehyde and further) in the electrolysis. As aresult, expensive solvent is lost and the aldehyde formed has to beseparated off in a complicated manner.

It is therefore an object of the invention to provide a process for theelectrochemical preparation of the γ-lactones and γ-hydroxycarboxylicesters by cross-coupling of α,β-unsaturated esters with carbonylcompounds, in which the disadvantages of the prior art, in particularthe use of divided electrochemical cells, of low current densities (<1A/dm²) and the occurrence of yield-reducing secondary reactions areavoided. This object is achieved by the claimed embodiments describedbelow.

The present invention accordingly provides a process for theelectrochemical preparation of γ-hydroxycarboxylic esters and/orγ-lactones by reductive cross-coupling of α,β-unsaturated esters withcarbonyl compounds in an undivided electrolysis cell, wherein thecathode material is selected from the group consisting of lead, leadalloys, cadmium, cadmium alloys, mercury, steel, glassy carbon andboron-doped diamonds and a basic, aqueous electrolyte comprising atleast one electrolyte salt selected from among bisquaternary andmultiquaternary ammonium and phosphonium salts is used.

For the purposes of the present invention, a carbonyl compound is analdehyde or a ketone, preferably an aldehyde. The carbonyl compoundsaccording to the invention preferably have a low solubility in water ofless than 100 g/l, more preferably less than 50 g/l, particularlypreferably less than 30 g/l, in each case at 20° C. Alkyl and/or arylgroups, which can also comprise further functional groups (for examplealcohol, ether, carbonyl, carboxylic acid groups, etc.) and can be alky,alkylene or arylene groups interrupted by oxygen, sulfur or nitrogen,are preferably bound to the carbonyl group of the carbonyl compounds.Particular preference is given to aliphatic carbonyl compounds which donot have any further heteroatoms in addition to the carbonyl group.Suitable carbonyl compounds are, for example, pentanal,2-methylpentanal, hexanal, 2-ethylhexanal, heptanal,4-formyltetrahydropyran, 4-methoxybenzaldehyde,4-tert-butylbenzaldehyde, 4-methylbenzaldehyde, glyoxal, glutaraldehyde,methylglyoxal, cyclohexenone, cyclohexanone, diethyl ketone.Particularly preferred carbonyl compounds are pentanal,2-methylpentanal, hexanal and heptanal.

For the purposes of the present invention, an α,β-unsaturated ester isan acrylic ester derivative which can be substituted independently inpositions 2 and 3, with two substituents also being possible in position3. The substituents are preferably alkyl groups, halogen atoms,C1-C20-alkoxy groups, alkyl, alkylene or arylene radicals interrupted byoxygen, sulfur or nitrogen, nitrile groups and nitro groups. Thesubstituents are preferably selected from the group consisting ofmethyl, ethyl, n-propyl, isopropyl, n-butyl, tent-butyl,trifluoromethyl, fluorine, chlorine, bromine, iodine, methoxy, ethoxy,methylene, ethylene, propylene, isopropylene, benzylidene, nitrile andnitro. Particular preference is given to substituents selected from thegroup consisting of methyl, ethyl, methoxy, ethoxy. The α,β-unsaturatedester is preferably a C1-C12-alkyl ester, particularly preferably aC1-C5-alkyl ester, very particularly preferably a methyl or ethyl ester.The α,β-unsaturated esters used according to the invention preferablyhave a low solubility in water of less than 100 g/l, preferably lessthan 50 g/l, particularly preferably less than 20 g/l, in each case at20° C.

α,β-Unsaturated esters and carbonyl compounds are the starting materialsfor the reductive coupling according to the invention.

An aqueous electrolyte for the purposes of the present inventioncomprises the starting materials together with water, at least oneelectrolyte salt and at least one buffer as components. In addition, theelectrolyte preferably also comprises at least one complexing agentand/or at least one anode corrosion inhibitor as further components. Theaqueous electrolyte in its totality with all components including thestarting materials will hereinafter also be referred to as reactionelectrolyte. The aqueous composition corresponding to the reactionelectrolyte without starting materials will hereinafter also be referredto as supporting electrolyte. The aqueous reaction electrolyte has awater content of preferably at least 20% by weight, particularlypreferably at least 50% by weight, in particular at least 75% by weight,based on the total aqueous reaction electrolyte.

The reaction electrolyte according to the invention comprises at leastone electrolyte salt, selected from among bisquaternary andmultiquaternary ammonium and phosphonium salts, which suppresses thecathodic formation of hydrogen. Preferably, apart from thesebisquaternary and multiquaternary ammonium and phosphonium salts, nofurther electrolyte salts are used. In general, the electrolyte salt isused in an amount in the range from 0.01 to 2.5% by weight, preferablyfrom 0.01 to 1.5% by weight, preferably from 0.01 to 0.5% by weight,particularly preferably from 0.05 to 0.25% by weight, based on the totalaqueous reaction electrolyte. Particularly suitable electrolyte saltsare bisquaternary ammonium and phosphonium salts (EP 635587 A).Particular preference is given to usingbis(dibutylethyl)hexamethylenediammonium hydroxide as electrolyte saltfor the electrolyte. Possible counterions are, for example, sulfate,hydrogensulfate, alkylsulfates, arylsulfates, alkylsulfonates,arylsulfonates, halides, phosphates, carbonates, alkyiphosphates,alkylcarbonates, nitrate, alkoxides, hydroxide, tetrafluoroborate orperchlorate. The acids derived from the abovementioned anions are alsopossible as electrolyte salts, i.e. for example sulfuric acid, sulfonicacids and carboxylic acids. Ionic liquids are also suitable aselectrolyte salts. Suitable ionic liquids are described in “IonicLiquids in Synthesis”, edited by Peter Wasserscheid, Tom Welton, VerlagWiley VCH, 2003, chapters 1 to 3, and also in DE 102004011427 A.

The reaction electrolyte further comprises at least one buffer having abuffering range at a pH of from 7 to 11, preferably from 8 to 10, forbuffering the protons formed in the anodic formation of oxygen. Suitablebuffers are, for example, hydrogenphosphate or hydrogencarbonate,preferably in the form of their sodium salts. Particular preference isgiven to using disodium hydrogenphosphate as buffer for the electrolyte.In general, the buffer is used in an amount in the range from 0.9 to 8%by weight, preferably from 4 to 7% by weight, based on the total aqueousreaction electrolyte.

Furthermore, the reaction electrolyte preferably comprises one or moreanode corrosion inhibitors such as the borates known for this purpose,preferably disodium diborate and orthoboric acid, in an amount of from0.4 to 3% by weight, preferably from 1 to 2% by weight, based on thetotal aqueous reaction electrolyte.

Furthermore, the reaction electrolyte preferably comprises one or morecomplexing agents in order to prevent the precipitation of iron and leadions. Mention may be made by way of example ofethylenediaminetetraacetate (EDTA), triethanolamine (TEA),triethylamine, nitrilotriacetate, preferably EDTA in an amount in therange from 0 to 1% by weight, preferably from 0.1 to 0.5% by weight,based on the total aqueous reaction electrolyte, and/or TEA in an amountin the range from 0 to 0.5% by weight, preferably from 0.05 to 0.2% byweight, based on the total aqueous reaction electrolyte. Instead of TEA,it is possible to use triethylamine in an amount of from 0 to 0.5% byweight, preferably from 0.05 to 0.2% by weight, based on the totalaqueous reaction electrolyte.

As anode material, it is possible to use known anode materials; in thecase of undivided cells, materials having a low oxygen overvoltage, forexample carbon steel, glassy carbon, steel, mercury, cadmium, platinum,iron, nickel, magnetite, lead, lead alloys or lead dioxide, are usuallyused. Preference is given to using an anode composed of steel, iron,lead or a lead alloy.

As cathodes, use is made of lead, lead alloys, cadmium, cadmium alloys,mercury, steel, glassy carbon or boron-doped diamond electrodes.Preference is given to using lead, lead alloys, cadmium, steel andglassy carbon as cathode materials. Particular preference is given tousing lead and lead alloys as cathode materials.

In the aqueous reaction electrolyte according to the invention, theorganic starting materials (α,β-unsaturated esters and carbonylcompounds) and the products formed (γ-hydroxycarboxylic esters andγ-lactones) are present as organic phase of an emulsion. The emulsion ismaintained during the electrolysis by mechanical agitation such asstirring or pump circulation of the electrolyte in the electrolysiscell, or else by addition of suitable emulsifiers which stabilize theemulsion. The emulsion is preferably maintained during the electrolysisby mechanical agitation such as stirring or pump circulation of theelectrolyte. After the electrolysis, demixing of the emulsion can beachieved, for example by stopping the agitation or by addition of asuitable flocculent. After demixing of the emulsion to get an aqueousphase and an organic phase, the products and any unreacted startingmaterials can easily be separated off with the organic phase from theaqueous electrolyte. This simplifies the separation of the products fromthe electrolyte.

In the electrolysis of the invention, the starting materialsα,β-unsaturated esters and carbonyl compounds are preferably used in anessentially equimolar ratio. The molar ratio of α,β-unsaturated esterused to carbonyl compound used is usually in the range from 0.25 to 4,preferably from 0.5 to 2, particularly preferably from 0.8 to 1.2. Whilean excess of carbonyl compound is used in the previously known processesfor reductive coupling of α,β-unsaturated esters with carbonyl compoundsin order to suppress the homo-coupling of the ester, the process of theinvention displays a high selectivity to the cross-coupling product ofα,β-unsaturated ester and carbonyl compound. When the starting materialsare used in an essentially equimolar ratio, particularly good yields ofthe cross-coupling product can be achieved by means of the process ofthe invention. The α,β-unsaturated ester is preferably used in an amountof from 1 to 25% by weight, particularly preferably from 5 to 10% byweight, based on the total aqueous reaction electrolyte.

The electrolysis is usually carried out at a current density of at least1 A/dm², preferably from 1 to 4 A/dm². However, it is also possible tocarry out the electrolysis at a higher current density of up to 20A/dm².

The electrolysis of the invention is usually carried out at atemperature of from 20 to 60° C. and under atmospheric pressure.

The electrolysis can be carried out either continuously or batchwise andin all conventional undivided electrolysis cells, for example in glassbeaker cells or plate cells and frame cells or cells having fixed-bed ormoving-bed electrodes. Both monopolar and bipolar connection of theelectrodes can be employed. The electrolyte in the electrolysis cell ispreferably circulated by pumping or stirred, as a result of which itspresence as emulsion can be maintained. Very particularly suitable cellsare capillary cells or plate stack cells connected in a bipolar manner,in which the electrodes are configured as plates and are arrangedparallel to one another (Ullmann's Encyclopedia of Industrial Chemistry,2009 electronic release, VCH-Verlag Weinheim, Volume Electrochemistry,Chapter 3, Electrochemical Cells and Chapter 5, OrganicElectrochemistry, Subchapter 5.4.3. Electrochemical Cells).

In an undivided electrolysis cell, anode space and cathode space are notseparated from one another by a membrane. Such undivided cells arecheaper and technically easier to release. Particularly in the case oforganic processes, the use of divided cells can lead to rapid aging ofthe membrane, which results in unsatisfactory operating lives.

In the process of the invention for the electrochemical reductive crosscoupling of α,β-unsaturated esters with carbonyl compounds, theγ-lactone or the corresponding γ-hydroxycarboxylic ester can in eachcase be formed either alone or as a mixture. If necessary, anyγ-hydroxycarboxylic ester formed can be converted into the γ-lactone bytransesterification after the electrochemical reductive cross-coupling.The transesterification to form the γ-lactone can, for example, becarried out by heating the γ-hydroxycarboxylic ester in the presence ofacid. If necessary, the alcohol liberated can be removed from thereaction mixture in order to shift the reaction in the direction of theγ-lactone. Conversely, any γ-lactone formed can be converted into theγ-hydroxycarboxylic ester by transesterification (alcoholysis), forexample by heating the γ-lactone in alkaline, nonaqueous alcoholicsolutions, after the electrochemical reductive cross-coupling. Theγ-hydroxycarboxylic ester can subsequently be converted further into thefree acid or the carboxylic acid salt by hydrolysis. For this purpose,the γ-hydroxycarboxylic ester is, for example, heated with aqueousalkaline solutions. As an alternative, the free γ-hydroxycarboxylic acidor its salt can also be prepared directly from the γ-lactone byhydrolysis. This can be carried out, for example, by heating theγ-lactone in aqueous, alkaline solutions.

The invention further provides the γ-butyrolactone derivatives of thegeneral formula I

where

R1, R2 and R3 are each, independently of one another, a hydrogen or analkyl group having from 1 to 5 carbon atoms, preferably a hydrogen, amethyl or ethyl group, and R4 and R5 are alkyl groups having from 1 to 4carbon atoms, preferably from 1 to 3 carbon atoms, with R4 and R5 beingidentical radicals,

which can be prepared by the process of the invention.

The compounds of the formula I can be prepared by the electrochemicalcross-coupling according to the invention of α,β-unsaturated esters ofthe formula II

with 2-alkylalkanals of the formula III

where R1 to R5 have the same meanings as in the compounds of the formulaI and R is an alkyl group, usually an alkyl group having from 1 to 12carbon atoms, preferably from 1 to 5 carbon atoms, very particularlypreferably a methyl or ethyl group.

The invention preferably provides the y-butyrolactone derivatives of thegeneral formula IV

where

R2 is a hydrogen or an alkyl group having from 1 to 5 carbon atoms,preferably a hydrogen, a methyl group or an ethyl group, and R4 and R5are alkyl groups having from 1 to 4 carbon atoms, preferably from 1 to 3carbon atoms, with R4 and R5 being identical radicals, which can beprepared by the process of the invention.

The compounds of the formula IV can be prepared by the electrochemicalcross-coupling according to the invention of α,β-unsaturated esters ofthe formula II (where R1 and R3 are in each case hydrogen) with2-alkylalkanals of the formula III.

The 2-alkylalkanals of the formula III can be prepared, for example, byaldol condensation of alkanals having from 3 to 6 carbon atoms(propanal, butanal, pentanal or hexanal).

Particular preference is given to the γ-butvrolactone derivatives4-(2-pentyl)butyrolactone

and 3-methyl-4-(2-pentyl)butyrolactone

which can be prepared by the electrochemical cross-coupling according tothe invention of acrylic esters or crotonic esters with2-methylpentanal.

The invention further provides the y-hydroxycarboxylic acids andy-hydroxycarboxylic esters of the general formula VIII

where

R1, R2, R3 and R7 are each, independently of one another, a hydrogen oran alkyl group having from 1 to 5 carbon atoms, preferably a hydrogen, amethyl group or an ethyl group, R is a hydrogen or an alkyl group,usually a hydrogen or an alkyl group having from 1 to 5 carbon atoms,and R8 is a branched alkyl group having from 3 to 10 carbon atoms, whichcan be prepared by the process of the invention.

The invention preferably provides the y-hydroxycarboxylic acids andy-hydroxycarboxylic esters of the general formula VIII, where R1, R2 andR3 are each, independently of one another, a hydrogen or an alkyl grouphaving from 1 to 5 carbon atoms, preferably a hydrogen, a methyl groupor an ethyl group, R is a hydrogen or an alkyl group, usually a hydrogenor an alkyl group having from 1 to 5 carbon atoms, R7 is a hydrogen andR8 is a branched alkyl group having from 3 to 10 carbon atoms, which canbe prepared by the process of the invention.

The compounds of the formula VIII can be prepared by the electrochemicalcross-coupling according to the invention of α,β-unsaturated esters ofthe formula II with the carbonyl compound of the formula VIII

where R7 and R8 have the same meanings as in the compounds of theformula VIII.

Alkyl groups for the purposes of the invention can in principle beeither branched or unbranched, either linear or cyclic and eithersaturated or unsaturated (including multiply unsaturated). Theypreferably have from 1 to 20, particularly preferably from 1 to 6,carbon atoms. They preferably do not have any heteroatoms.

Aryl groups for the purposes of the invention are aromatic radicalshaving preferably from 5 to 20 carbon atoms.

The invention further provides for the use of the γ-butyrolactonederivatives of the formula I according to the invention, preferably theγ-butyrolactone derivatives of the formula IV, particularly preferably4-(2-pentyl)butyrolactone or 3-methyl-4-(2-pentyl)butyrolactone asfragrances or flavors. 4-(2-Pentyl)butyrolactone has a pear-like aromaand 3-methyl-4-(2-pentyl)butyrolactone has a wood-like aroma.

EXAMPLES

The invention will now be illustrated by the following, nonlimitingexamples.

Example 1

Electrochemical preparation of ethyl y-hydroxypelargonate and4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate withhexanal using an excess of hexanal

Ethyl acrylate (1.7% by weight) and hexanal (29.7% by weight) wereemulsified in an aqueous electrolyte (0.16% by weight ofbis(dibutylethyl)hexamethylenediammonium hydroxide (bisquat), 0.38% byweight of EDTA, 0.14% by weight of TEA, 1.45% by weight of Na2B4O7 and5.84% by weight of Na₂HPatin water at a pH of 10) (all % by weight arebased on the total aqueous reaction electrolyte) and subjected togalvanostatic electrolysis at a current density of 2.23 A/dm² and atemperature of 20° C. in a pot cell. The current throughput was 2 F/molof ester. A steel anode and a lead cathode were used as electrodes(electrode area of 0.1 dm² and spacing of 1 cm). To monitor thereaction, the methyltributylammonium methylsulfate extract (MTBEextract) of a sample of the electrolysis output was analyzed by gaschromatography. A yield of 0.1% of 4-pentylbutyrolactone and a yield of2.9% of the corresponding ethyl γ-hydroxypelargonate were achieved. Thiscorresponded to a total yield of target products 3.0% of the theoreticalyield.

Example 2

Electrochemical preparation of ethyl γ-hydroxypelargonate and4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate withhexanal using the starting materials in an equimolar ratio

Ethyl acrylate (5.9% by weight) and hexanal (6.0% by weight) werereacted (all % by weight are based on the total aqueous reactionelectrolyte) and analyzed as described in example 1. A yield of4-pentylbutyrolactone of 23.7% and a yield of the corresponding ethylγ-hydroxypelargonate of 48.0% were achieved. This corresponded to atotal yield of target products of 71.7% of the theoretical yield.

Comparative Example 1

Electrochemical preparation of ethyl y-hydroxypelargonate and4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate withhexanal using an excess of hexanal

Corresponding to the reductive coupling described by Burger, ethylacrylate (1.7% by weight) and hexanal (29.7% by weight) were dissolvedin an electrolyte (17.0% by weight of tetrabutylamine tetrafluoroborate(Buar\lBF4) in a 3:1 mixture of dioxane and ethanol)(all % by weight arebased on the total aqueous reaction electrolyte) and subjected togalvanostatic electrolysis at a current density of initially 2.23 A/dm²and a temperature of 21° C. in a pot electrolysis cell. The currentthroughput was 2 F/mol of ester. During the course of the electrolysis,the current density dropped to 0.73 A/dm². A platinum anode and agraphite cathode were used as electrodes (electrode area of 0.1 dm² andspacing of 1 cm). To monitor the reaction, the methyltributylammoniummethylsulfate extract of a sample of the electrolysis output wasanalyzed by gas chromatography. A yield of 4-pentylbutyrolactone of 2.0%and a yield of the corresponding ethyl y-hydroxypelargonate of 0.2% wereachieved. This corresponded to a total yield of target product of 2.2%of the theoretical yield.

Comparative Example 2

Electrochemical preparation of ethyl y-hydroxypelargonate and4-pentylbutyrolactone by reductive cross-coupling of ethyl acrylate withhexanal using the starting materials in an equimolar ratio

Ethyl acrylate (5.9% by weight) and hexanal (6.0% by weight) werereacted at 22° C. (all % by weight are based on the total aqueousreaction electrolyte) and subsequently analyzed as described incomparative example 1, with the current density remaining constantduring the experiment. A yield of 4-pentylbutyrolactone of 16.5% and ayield of the corresponding ethyl γ-hydroxypelargonate of 0.7% wereachieved. This corresponded to a total yield of target products of 17.2%of the theoretical yield.

Example 3

Preparation of Whiskey Lactone

Ethyl crotonate (6.9% by weight) and pentanal (5.2% by weight) wereemulsified in an aqueous electrolyte (0.16% by weight ofbis(dibutylethyl)hexamethylenediammonium hydroxide (bisquat), 0.38% byweight of EDTA, 0.14% by weight of TEA, 1.45% by weight of Na2B4O7 and5.84% by weight of Na2HPO4 in water at a pH of 10) (all % by weightbased on the total aqueous reaction electrolyte) and subjected togalvanostatic electrolysis at a current density of 2.23 A/dm² and atemperature of 25° C. in a frame electrolysis cell. The currentthroughput was 2 F/mol of ester. A steel anode and a lead cathode wereused as electrodes (electrode area of 0.1 dm² and spacing of 1 cm). Tomonitor the reaction, the MTBE extract of a sample of the electrolysisoutput is analyzed by gas chromatography. A yield of3-methyl-4-butylbutyrolactone (whiskey lactone) of 68.5% and a yield ofthe corresponding ethyl y-hydroxycarboxylate of 24.2% were achieved.This corresponded to a total yield of target products of 92.7% of thetheoretical yield.

Example 4

Preparation of 4-(2-pentyl)butyrolactone

Ethyl acrylate (5.9% by weight) and 2-methylpentanal (6.0% by weight)were reacted electrochemically using a method analogous to example 3(all % by weight are based on the total aqueous reaction electrolyte). Ayield of 4-(2-pentyl)butyrolactone of 88.4% was achieved.

Example 5

Preparation of 3-methyl-4-(2-pentyl)butyrolactone

Ethyl crotonate (6.7% by weight) and 2-methylpentanal (5.9% by weight)were reacted electrochemically by a method analogous to example 3 (all %by weight are based on the total aqueous reaction electrolyte). A yieldof 4-(2-pentyl)butyrolactone of 66.7% and a yield of the correspondingethyl y-hydroxycarboxylate of 26.6% were achieved. This corresponded toa total yield of target products of 93.2% of the theoretical yield.

Example 6

Preparation of 3,3-dimethyl-4-pentylbutyrolactone

Methyl 3,3-dimethylacrylate (4.9% by weight) and hexanal (4.3% byweight) were reacted electrochemically by a method analogous to example3 (all % by weight are based on the total aqueous reaction electrolyte).A yield of 3,3-dimethyl-4-pentylbutyrolactone of 37.2% was achieved.

Example 7

Preparation of 2-methyl-4-butylbutyrolactone

Ethyl methacrylate (6.7% by weight) and pentanal (5.2% by weight) werereacted electrochemically using a method analogous to example 3 (all %by weight are based on the total aqueous reaction electrolyte). A yieldof 2-methyl-4-butylbutyrolactone of 81.0% was achieved.

Example 8

Preparation of 2-methyl-4-(2-pentyl)butyrolactone

Ethyl methacrylate (6.7% by weight) and methylpentanal (5.9% by weight)were reacted electrochemically using a method analogous to example 3(all % by weight are based on the total aqueous reaction electrolyte). Ayield of 2-methyl-4-(2-pentyl)butyrolactone of 70.9% was achieved.

1. A γ-butyrolactone derivative of formula I

wherein R1, R2 and R3 are each independently a hydrogen or an alkylgroup having from 1 to 5 carbon atoms,. and R4 and R5 are alkyl groupshaving from 1 to 4 carbon atoms, with R4 and R5 being identicalradicals.
 2. The γ-butyrolactone derivative according to claim 1,wherein the derivative is selected from the group consisting of4-(2-pentyl)butyrolactone and 3 -methyl-4-(2-pentyl)butyrolactone. 3.The γ-butyrolactone derivative according to claim 1, wherein thederivative is a flavor.
 4. A γ-hydroxycarboxylic acid orγ-hydroxycarboxylic ester of formula VII

wherein R1, R2, R3 and R7 are each independently, a hydrogen or an alkylgroup having from 1 to 5 carbon atoms, R is a hydrogen or an alkylgroup, and R8 is a branched alkyl group having from 3 to 10 carbonatoms.
 5. The γ-hydroxycarboxylic acid or γ-hydroxycarboxylic esteraccording to claim 4, wherein R7 is a hydrogen.